1. Field of the Invention
The present invention relates to a common-mode choke coil having a core and wire, and in particular, relates to a common-mode choke coil suitable for use in a common-mode filter for eliminating noise.
2. Description of the Related Art
In general, a wire-wound common-mode choke coil is known as a necessary radiant noise relief measure in a power supply line or as a common-mode noise relief measure of a high-frequency signal (Japanese Unexamined Patent Application Publication No. 2000-133522, for example). Such a common-mode choke coil utilizing a conventional technique includes a ferrite magnetic core with a winding core having flanges at both ends, a plurality of pieces of wire made of insulating coated-copper wire and wound around the winding core by several turns to several dozen turns in a bifilar winding manner, a magnetic shielding member (flat-plate core) connected between both the flanges of the magnetic core and having substantially the same permeability as that of the magnetic core. Also, in the both flanges or one of the flanges, a plurality of electrodes are arranged so as to be conductively connected to ends of the wire, the tongue and tail, by soldering or thermo-compression. In such a common-mode choke coil, a desired impedance value is obtained by appropriately setting the number of turns of the wire wound around the winding core.
Meanwhile, a new standard of an interface USB (Universal Series Bus) 2.0 between a personal computer and peripheral equipment has been introduced and is being used, so that the development of personal computers and peripheral equipment conforming to this standard is very active. The data-transmitting rate of the USB 2.0 is 480 Mbps, which is extremely high. Therefore, in designing the personal computers and peripheral equipment conforming to the interface USB 2.0 so as to deal with a signal line with a high frequency of 480 MHz, it is necessary to consider the distortion of a signal waveform and to particularly consider the prevention of radiant magnetic noise. As a result, in wiring on a main board of a personal computer, for example, it is necessary that the mismatching in impedance be eliminated, and common-mode noise produced by the signals transmitting in a plurality of pieces of wire in the same direction be prevented to minimize radiant magnetic noise in connectors.
In conforming to the pre-existing interface USB 1.1, a ferrite bead inductor is generally used for suppressing radiant magnetic noise in connectors. However, since the applicable frequencies of the ferrite bead inductor range up to substantially several dozen MHz, in the USB 2.0 operating at 480 MHz, the high-frequency component of a signal waveform is rather reduced. Therefore, in designing personal computers and peripheral equipment conforming to the interface USB 2.0, it is recommended to use a common-mode choke coil as a common-mode noise relief measure instead of the ferrite bead inductor.
However, because the applicable frequencies of a conventional common-mode choke coil are about several MHz to about 200 MHz, high-frequency common-mode noise in the interface USB 2.0 ranging from several hundred MHz to several GHz cannot be eliminated. Accordingly, the improvement of frequency characteristics in a common-mode impedance has been intensively demanded to enable the common-mode noise having a bandwidth of several hundred MHz or more to be eliminated.
On the other hand, in the use of a common-mode choke coil having high common-mode impedance characteristics for eliminating the common-mode noise having a bandwidth of several hundred MHz or more, the possibility of not satisfying EOP (End-Of-Packet) and an eye pattern in the USB code may exist.
In order to satisfy the EOP and the eye pattern in the USB code, the common-mode impedance must be reduced, and specifically, for satisfying the EOP and the eye pattern, a common-mode impedance of about 120 Ω or less (the optimum value is 90 Ω) is required for a signal of 100 MHz, for example. Whereas, in the conventional wire-wound common-mode choke coil, the number of turns of the wire can be only set to be an integral multiple of one turn, and the values of impedance are discontinuous, so that a desired impedance value may not be obtained. For example, in a coil having wire wound by three turns, an impedance of about 67 Ω is obtained for a signal of 100 MHz, whereas in a coil having wire wound by four turns, an impedance of about 120 Ω is obtained for a signal of 100 MHz, so that there is a difference of about 50 Ω therebetween. At this time, there has been a problem that the EOP and the eye pattern cannot be satisfied at 120 Ω while noise eliminating characteristics cannot be secured at 67 Ω.
Also, in a structure in which a magnetic shielding member having substantially the same permeability as that of the magnetic core is connected between the flanges of the magnetic core, the impedance value may change because of variations in the clearance size between the magnetic core and the magnetic shielding member or the displacement of the magnetic shielding member, so that there has also been a problem that the EPO and eye pattern are not satisfied.